The present invention is directed to binders for use in the formation of nonwoven mats to be utilized in areas where heat resistance is important. Such mats find use in a variety of applications including as components in roofing, flooring and filtering materials.
Specifically, in the formation of asphalt-like roofing membranes such as those used on flat roofs, polyester mats about 1 meter in width are formed, saturated with binder, dried and cured to provide dimensional stability and integrity to the mats allowing them to be rolled and transported to a converting operation where one or both sides of the mats are coated with molten asphalt. The binder utilized in these mats plays a number of important roles in this regard. If the binder composition does not have adequate heat resistance, the polyester mat will shrink when coated at temperatures of 170.degree.-250.degree. C. with the asphalt. A heat resistant binder is also needed for application of the roofing when molten asphalt is again used to form the seams and, later, to prevent the roofing from shrinking when exposed to elevated temperatures over extended periods of time. Such shrinking would result in gaps or exposed areas at the seams where the roofing sheets are joined as well as at the perimeter of the roof.
Since the binders used in these structures are present in substantial amounts, i.e., on the order of about 25% by weight, the physical properties thereof must be taken into account when formulating for improved heat resistance. Thus, the binder must be stiff enough to withstand the elevated temperatures but must also be flexible at room temperature so that the mat may be rolled or wound without cracking or creating other weaknesses which could lead to leaks during and after impregnation with asphalt.
Binders for use on nonwoven mats have conventionally been prepared from acrylate or styrene/acrylate copolymers. In order to improve the heat resistance thereof, crosslinking functionalities including N-methylol containing comonomers, have been incorporated into these copolymers; however, the addition of more than about 3% by weight of the N-methylol component is difficult to achieve due to thickening of the latex, particularly those latices containing styrene, at the 45 to 60% solids level most commonly used.
Other techniques for the production of heat resistant roofing materials include that described in U.S. Pat. No. 4,539,254 involving the lamination of a fiberglass scrim to a polyester mat thereby combining the flexibility of the polyester with the heat resistance of the fiberglass.